Ceramic cores are used to form cooling cavities and passages within airfoil portions of buckets and nozzles used in the hot section of a gas turbine. Typically, the cooling passages in, for example, a turbine stage one, and sometimes stage two, bucket form a serpentine shape. This serpentine geometry usually includes 180.degree. turns at both the root and the tip of the airfoil. The turns at the tip end of the airfoil are generally well supported outside of the airfoil. The turns at the root, on the other hand, are generally supported by cross-ties of small conical (or similar) geometry, which attach at one end to the root turns and at the opposite end to the coolant supply and/or exit passages in the turbine bucket shank. Thus, the ceramic core is essentially a solid body which is shaped to conform to the complex interior coolant passages of the bucket. The core is placed within a casting mold prior to pouring of molten metal into the mold to form the bucket. A casting mold which holds the core consists of a ceramic shell which contains the molten metal, forms the exterior shape of the component, and fixes the ceramic core within the part being cast.
Ceramic cores are formed by creating a die of the cooling circuit geometry into which a slurry of the desired composition is injected. The "green" material is then fired to cure the ceramic, making the core stable and rigid. Of course, the geometry and conditions to which the ceramic core are exposed in the casting mold are important considerations in maintaining the structural stability of the core. For example, airfoil lengths for certain gas turbine nozzles and buckets for which the cooling geometry require core stability, range from approximately six inches to twelve inches and longer. Typically, ceramic core compositions have been formulated to achieve structural integrity under moderately high temperatures for extended lengths of time. During casting, however, the ceramic core is exposed to molten metal which can be as hot as 2700.degree. F. Directional solidification of the metal, for example, producing either columnar or single crystal grain structures, requires very slow withdrawal rate from the furnace. This slow rate exposes the ceramic core to very high temperatures for extended periods of time. The ceramic core tends to lose its structural stability under these conditions, and deforms due to its own weight. This phenomenon, known as "slumping", causes undesirable variations in the final product's wall thickness between the mold and the core. The problem has been linked to the use of more advanced nickel-base superalloys with hotter pouring temperatures and longer withdrawal times.
There are certain ceramic compositions, however, which, upon a non-reversible phase change, produce extremely hard and stable structures with minimal slumping during casting. The difficulty with these compositions, however, is that the normal core removal process (high temperature leaching baths) does not work well. Since leaching represents the only non-destructive core removal technique available, there is no viable process to remove the hard stable cores from the casting.